The overall goal of this project is to explore cellular mechanism of protection against radiation-induced programmed cell death (apoptosis) in endothelial and other non-lymphoid mammalian cells, and to investigate the signal transduction pathways of these effects. Based on our recent experiments, we propose the hypothesis that apoptosis represents a mode of inter-mitotic lethal effect of radiation, regularly induced in mammalian cells after radiation exposure, and that the so-called "SLDR" and "PLDR" represent natural rescue mechanisms that down-regulate this process. We further hypothesize that apoptosis is just one of the pleiotropic lethal effects of ionizing irradiation on mammalian cells, which also includes postmitotic death from unrepaired DNA double strand breaks and chromosomal aberrations. The relative contribution from each mode of cell death may differ with dose and differs from one cell type to another, relative to their inherent and inducible capacities to overcome each of these types of radiation damage. The scope of this research proposal has been restricted to several key issues related to the mechanisms of radiation-induced apoptosis and its prevention in endothelial and several other non-lymphoid mammalian cells. The specific aims of the project are to assess quantitative parameters of radiation- induced apoptosis in these cell systems, study its signal transduction pathway, and characterize SLDR and PLDR in relation to their anti- apoptotic effects. Experiments will be carried out to evaluate dose- response and time course relationships in the progression of apoptosis after radiation exposure to establish a base-line for the mechanistic experiments. Recent investigations have shown that the newly discovered sphingomyelin pathways is involved in the transduction of early apoptotic signals in TNF-treated cells. Experiments will be carried out to investigate the role of this signaling pathway in the transduction of the radiation-induced apoptosis. In addition, the levels and sequence of expression of several other gene products, known to be expressed during apoptosis in other systems, will also be studied. To study the mechanism of inhibition of radiation-induced apoptosis inhibition of apoptosis and the clonogenic survival during the progression of SLDR and PLDR. The time dependent changes in membrane PKC activity after an exposure to radiation will be correlated with the time-fractionated irradiation. An improved understanding of the mechanisms of radiation-induced apoptosis, SLDR and PLDR may provide new opportunities to intervene in specific pathways of radiation damage and repair to favorable alter the therapeutic ratio in radiation treatments of human cancer.